CavusogluPBU20123MCavusogluRPohmannHCBurgerKUludag2013-02-00269524–530Magnetic Resonance in MedicineMost experiments assume a global transit delay time with blood flowing from the tagging region to the imaging slice in plug flow without any dispersion of the magnetization. However, because of cardiac pulsation, nonuniform cross-sectional flow profile, and complex vessel networks, the transit delay time is not a single value but follows a distribution. In this study, we explored the regional effects of magnetization dispersion on quantitative perfusion imaging for varying transit times within a very large interval from the direct comparison of pulsed, pseudo-continuous, and dual-coil continuous arterial spin labeling encoding schemes. Longer distances between tagging and imaging region typically used for continuous tagging schemes enhance the regional bias on the quantitative cerebral blood flow measurement causing an underestimation up to 37% when plug flow is assumed as in the standard model.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published-524Regional effects of magnetization dispersion on quantitative perfusion imaging for pulsed and continuous arterial spin labeling1501718821CavusogluBYU20113MCavusogluABartelsBYesilyurtKUludag2012-02-004594044–4054NeuroImageCortical representations of the visual field are organized retinotopically, such that nearby neurons have receptive fields at nearby locations in the image. Many studies have used blood oxygenation level-dependent (BOLD) fMRI to non-invasively construct retinotopic maps in humans. The accuracy of the maps depends on the spatial extent of the metabolic and hemodynamic changes induced by the neural activity. Several studies using gradient-echo MRI at 1.5 T and 3 T showed that most of the BOLD signal originates from veins, which might lead to a spatial displacement from the actual site of neuronal activation, thus reducing the specificity of the functional localization. In contrast to BOLD signal, cerebral blood flow (CBF) as measured using arterial spin labeling (ASL) is less or not at all affected by remote draining veins, and therefore spatially and temporally more closely linked to the underlying neural activity. In the present study, we determined retinotopic maps in the human brain using CBF as well as using BOLD signal in order to compare their spatial relationship and the temporal delays of each imaging modality for visual areas V1, V2, V3, hV4 and V3AB. We tested the robustness and reproducibility of the maps across different sessions, calculated the overlap as well as signal delay times across visual areas. While area boundaries were relatively well preserved, we found systematic differences of response latencies between CBF and the BOLD signal between areas. In summary, CBF data obtained using ASL allows reliable retinotopic maps to be constructed; this approach is, therefore, suitable for studying visual areas especially in close proximity to large veins where the BOLD signal is spatially inaccurate.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published-4044Retinotopic maps and hemodynamic delays in the human visual cortex measured using arterial spin labeling1501718821150171542156893MCavusogluJPfeufferKUgurbilKUludag2009-10-0082710391045Magnetic Resonance ImagingArterial spin labeling (ASL) using magnetic resonance imaging (MRI) is a powerful noninvasive technique to investigate the physiological status of brain tissue by measuring cerebral blood flow (CBF). ASL assesses the inflow of magnetically labeled arterial blood into an imaging voxel. In the last 2 decades, various ASL sequences have been proposed which differ in their ease of implementation and their sensitivity to artifacts. In addition, several quantification methods have been developed to determine the absolute value of CBF from ASL magnetization difference images. In this study, we evaluated three pulsed ASL sequences and three absolute quantification schemes. It was found that FAIR-QUIPSSII implementation of ASL yields 1020% higher signal-to-noise ratio (SNR) and 18% higher CBF as compared with PICORE-Q2TIPS (with FOCI pulses) and PICORE-QUIPSSII (with BASSI pulses). In addition, quantification schemes employed can give rise to up to a 35% difference in CBF values. We conclude that, although all quant
itative ASL sequences and CBF calibration methods should in principle result in the similar CBF values and image quality, substantial differences in CBF values and SNR were found. Thus, comparing studies using different ASL sequences and analysis algorithms is likely to result in erroneous intra- and intergroup differences. Therefore, (i) the same quantification schemes should consistently be used, and (ii) quantification using local tissue proton density should yield the most accurate CBF values because, although still requiring definitive demonstration in future studies, the proton density of blood is assumed to be very similar to the value of gray matter.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published6Comparison of pulsed arterial spin labeling encoding schemes and absolute perfusion quantification1501718821CavusogluPU20127MCavusogluRPohmannKUludagMelbourne, Australia2012-05-0920th Annual Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine (ISMRM 2012)The arterial transit time is a key local variable which has to be included in the kinetic models to estimate the CBF (f) in arterial spin labeling. Due to cardiac pulsation, non-uniform cross-sectional flow profile and complex vessel networks, the distribution of transit delay time has a statistical nature instead of being uniform. In this study, we have investigated the regional effects of magnetization dispersion for varying distances between tagging and imaging regions by implementing pulsed (PASL), pseudo-continuous (PCASL) and dual-coil continuous (DC-CASL) ASL encoding schemes.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published0Magnetization dispersion effetcs on quantitative perfusion imaging for pulsed and continuous arterial spin labeling1501718821CavusogluBU20127MCavusogluABartelsKUludagMelbourne, Australia2012-05-0057820th Annual Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine (ISMRM 2012)nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published-578Retinotopic maps and hemodynamic delays in the human visual cortex measured using arterial spin labeling15017188211501715421CavusogluYBU20107MCavusogluBYesilyurtABartelsKUludagBarcelona, Spain2010-06-0016th Annual Meeting of the Organisation for Human Brain Mapping (HBM 2010)nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published0Retinotopy of the human visual cortex with perfusion contrast
using arterial spin labeling1501718821150171542156867MCavusogluKUgurbilKUludagToronto, Canada2008-05-0532416th Scientific Meeting and Exhibition of the International Society of Magnetic Resonance in Medicine (ISMRM 2008)Three different ASL schemes (Q2TIPS, FAIR-QUIPSSII and PICORE-QUIPSSII with asymmetric BASSI pulses) were compared measuring absolute cerebral blood flow (CBF) in the human brain at 3T. Signal-to-noise ratios (SNR), magnetization difference between control and tag images and three different absolute CBF quantification schemes were evaluated. For identical sequence parameters and voxels, it was found that a) FAIR-QUIPSSII has the highest SNR b) FAIR-QUIPSSII yields slightly higher CBF values c) absolute CBF values depend on the quantification scheme utilized. One possible explanation for the findings is that effective labeling efficiency and physiological noise contamination are different for the tagging procedures used.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/fileadmin/user_upload/files/publications/ISMRM-2008-01930.pdfpublished-324Comparison of Pulsed Arterial Spin Labeling Sequences Using Different Absolute Quantification Methods1501718821YazdanbakhshRCHL201310PYazdanbakhshFResmerMCavusogluAHenningTLanz708210MCavusogluCavusoglu200910MCavusoglu